Wavelength division multiplexed (WDM) optical communication systems are known in which multiple optical signals, each having a different wavelength, are combined onto a single optical fiber. Such systems typically include a laser associated with each wavelength, a modulator configured to modulate the output of the laser, and an optical combiner to combine each of the modulated outputs. Conventional WDM systems have been constructed from discrete components. For example, the lasers, modulators and combiners have been packaged separately and mounted on a printed circuit boards. More recently, however, many WDM components including transmitters, receivers and passive devices have been integrated onto a single chip also referred to as a photonic integrated circuit (PIC).
Optical signals output from a PIC are typically modulated to carry data. For example, in the common RZ modulation format, an optical pulse having an optical power that exceeds a threshold value for a particular duration represents, for example, a logical 1 bit. On the other hand, a pulse having an optical power less than the threshold represents a logical 0 bit, for example.
In order to further increase data transmission rates associated with WDM systems, other modulation formats have been proposed for generating a modulated output. One such modulation format, known as polarization multiplexed differential quadrature phase-shift keying (“Pol Mux DQPSK”), can provide higher data rates as compared to the RZ modulation format. Thus, transmitter PICs that output Pol Mux DQPSK or other so-called “advanced modulation format” signals have been developed that include more components and greater complexity than PICs that supply RZ formatted signals, for example. Packaging of advanced modulation format PICs is also rendered more complicated, since the package should preferably have a reduced size, but accommodate an increased number of connections or pins. Advantageously, the package housing an advanced modulation format PIC, for example, should also facilitate simplified testing of the PIC.